To ensure the continued operational capability of the fluid containing vessels used in a nuclear power plant facility, the integrity of these vessels is periodically tested. Such vessels include a pressure vessel and a perpendicularly-oriented nozzle welded to the pressure vessel which communicates with the interior of the pressure vessel.
The Nuclear Regulatory Commission, under authority granted by the Congress of the United States, establishes rules and regulations for the operation of domestic nuclear facilities. These rules, and amendments to the rules, are publicized in the Federal Register under 10CFR50, Industry Codes and Standards. The Commission has established the American Society of Mechanical Engineers (ASME) Boiler & Pressure Vessel Code as the engineering authority for the design, construction, and operation of nuclear reactors. Section XI of the ASME Code (incorporated by reference herein as if fully written out below) contains the rules for in-service inspection of nuclear plant components as amended by 10CFR50.55a (Final Rule).
ASME Section XI, Article IWA-1320 (a) (1) states “the rules of IWB shall be applied to those systems whose components are classified ASME Class 1 (Quality Group A).
ASME Section XI, Subsection IWB provides the requirements for Class 1 components of light-water cooled plants and states in Article IWB-2000 that “Examinations required by this Article shall be completed prior to initial plant start-up”up”. It states that the sequence of component examinations, which was established during the first inspection interval, shall be repeated during each successive inspection interval, to the extent practical. Components shall be examined and tested as specified in Table IWB-2500-1, which specifically defines examination category B-D, Full Penetration Welded Nozzles in Vessels. This category includes the “Nozzle Inside Radius Section”. The examination method required is volumetric, which is either an ultrasonic technique or a radiographic technique. FIG. 1 areas “A” and “B” define the Nozzle Inside Radius Section or examination area, ts is vessel thickness, and tn1 is nozzle boss thickness. The beam 112 of the transducer probe 111 is pointed in the direction of areas “A” and “B” which are situated between the nozzle 12 and pressure vessel 14. FIG. 1 also shows the weld 110 between the nozzle 12 and the pressure vessel 14.
ASME Section XI, Article IWA-2000, which stipulates requirements for Examination and Inspection, covers general requirements, examination methods, qualifications of nondestructive examination personnel, inspection program, extent of examination and weld reference system. Under IWA-2200 “Examination Methods” is sub article IWA-2230, Volumetric Examination and under that is IWB-2232, Ultrasonic Examination, which states that “Ultrasonic examination shall be conducted in accordance with Appendix 1.
ASME Section XI, Appendix 1, Article I-2000 stipulates what examination requirements are required for each type of component. The requirement for Vessels Greater than 2 in. (51 mm) in Thickness are found in I-2110(a) Ultrasonic examination procedures, equipment, and personnel used to detect and size flaws in reactor vessels greater than 2 in. (51 mm) in thickness shall be qualified by performance demonstration in accordance with Appendix VIII for the following specific examinations and no other I-2000 requirements apply.
(1) Shell and Head Welds Excluding Flange Welds
(2) Nozzle to Vessel Welds
(3) Nozzle Inside Radius Section
(4) Clad/Base Metal Interface Region
ASME Section XI, Appendix VIII, Performance Demonstration for Ultrasonic Examination Systems, Article VIII-3000, Qualification Requirements, states that examination procedures, equipment and personnel are qualified for both detection and sizing flaws upon successful completion of the performance demonstration specified in the appropriate Supplement listed in Table VIII-3110-1, Supplement 5 being Nozzle Inside Radius Section.
In addition to the pressure vessel and nozzle themselves, an area of concern is the blend formed there between, i.e., the nozzle's inside radius section. The blend refers to the welded interface between the pressure vessel and nozzle. Because the pressure vessel and nozzle have cylindrical shapes, the shape of the blend is contingent on the relative diameters of the pressure vessel and nozzle. For example, if the pressure vessel has a significantly larger diameter than the nozzle, then the blend (for a vertically oriented pressure vessel) is slightly bowed between its vertical extremities. However, if the pressure vessel and nozzle have identical diameters then the blend (for a vertically oriented pressure vessel) is significantly bowed between its vertical extremities.
Because the pressure vessel normally has a significantly larger diameter than the nozzle, the shape of the blend associated therewith is only slightly bowed. Nevertheless, the blend has a complex three-dimensional geometry when compared to the pressure vessel and nozzle. To insure accurate testing of the fluid containing vessels, the exterior and interior dimensions of the nozzle, pressure vessel, and blend are recorded before the nuclear facility begins operation.
In putting together the Supplement 5, demonstration requirements, 10CFR50.55a allowed an alternative method to be used, Code Case N-552, “Qualification for Nozzle Inside Radius Section from the Outside Surface”. With regard to flaws 113 this Code Case requires that a model be used to calculate the incident angle 114, misorientation angle 115, and the maximum metal path distance to the required inspection volume, which is on the inside surface, wherein Ts is surface tangent, Ns is surface normal and Nf is flaw normal. There is an additional requirement to calculate angle at the flaw 116 (nominal inspection angle), also on the inside surface. These are referred to as essential parameters of a nozzle inside radius examination and are shown in FIG. 2.
The blend dimensions are translated into a three-dimensional computer model which is used for testing of the operational capability of the nozzle, pressure vessel, and blend. In fact, since the nozzle, pressure vessel, and blend are located in a radiation area, and access to the interior surface thereof is restricted during operation, the computer model is used for testing the integrity of the interior surface. To that end, a computerized testing program is used to develop a testing regime according to the computer model that specifies the procedures necessary to test the integrity of the interior surface of the nozzle, pressure vessel, and blend during operation of the nuclear facility.
Such a testing regime uses various transducer probes to determine whether there are flaws such as cracks, voids or slag build-up on the interior surface of the nozzle, pressure vessel, and blend. Since extended exposure to the radiation area in which the nozzle, pressure vessel, and blend are located is a consideration, the testing regime is configured to limit the number of testing iterations, and maximize the coverage (i.e. the amount of the interior surface area analyzed) for each iteration.
A computational model is required due to the complexity of the different nozzle geometries, in order to achieve 100% coverage of the nozzle inside radius area, which is designated as the area between bore S=0 and vessel S=Smax in FIG. 3, nozzle cross section. Normally the examination requires scanning from the vessel outer shell radius “Rvo” (vessel inner shell radius “Rvi”) and from the blend outer radius “Rbo” (blend inner radius “Rbi”) with several different transducer angles and skews, wherein R on the x axis is the distance from the nozzle center and Z on the y axis is the distance from the vessel center.
To illustrate, a testing regime may specify three iterations each using a differently-angled transducer probe. Each iteration will have an approximately cylindrical surface area around the nozzle, pressure vessel, and blend associated therewith where a specified transducer probe is utilized. During each iteration, the specified transducer probes are manually moved by a technician three-hundred-sixty degrees (360E) around the associated, approximately cylindrical surface areas.
The approximately cylindrical surface areas for each iteration are defined between two rings spaced around the exterior surface of the nozzle, pressure vessel, and blend. To insure complete coverage, the approximately cylindrical surface areas for the three iterations may overlap.
To further insure complete coverage, the testing regime also provides a range of skews (i.e. rotational orientations) at which the specified transducer probe for each iteration are to be oriented as they are moved around the exterior surface of the nozzle, pressure vessel, and blend.
Each of the transducer probes used during the various iterations are calibrated to excite a signal reflection as they are moved around the exterior surface of the nozzle, pressure vessel, and blend. These signal reflections correspond to flaws, the aforementioned cracks, voids or slag build-up, on the interior surface of the nozzle, pressure vessel, and blend. Upon recognition of the receipt of a signal reflection, the position of the transducer probe on the exterior surface is indicated by the technician. Conventionally, the technician performing the test indicates the location and position of the transducer probe, such as by tracing the outline of the transducer probe on the exterior surface of the nozzle, pressure vessel, and blend, or by any other suitable marking or tagging technique.
After the various iterations specified by the testing regime are completed, the positions of the flaw indications (such as traced outlines signifying the location and position of the transducer probes when the signal reflections were received) are entered into the computerized testing program. Given the angle of the transducer probe utilized, and the coordinates and skew of the transducer probe when the signal reflection was received, the computerized testing program (using the above-discussed three-dimensional computer model) is capable of mapping the position of the flaw associated with the signal reflection on the interior surface of the nozzle, pressure vessel, and blend. Once the flaw is located, the significance of the flaw can be evaluated to determine the operational capability of the vessels.
The definition of skew is shown in FIGS. 4a-d. With a 0° skew aligned with the nozzle 12 axis, beam 112 of the transducer probe 111 pointed in the direction of the blend 16 and nozzle 12 center as shown in FIG. 4a; 90° skew is with the beam 112 pointed circumferentially around the nozzle 12 in either the clockwise (+90) direction as shown in FIG. 4d or counter-clockwise (−90) direction as shown in FIG. 4b; and 180° skew is again aligned with the nozzle 12 axis but the beam 112 is pointed in the direction of the vessel shell as shown in FIG. 4c. 
When an indication is recorded during an in-service examination, to accurately locate the flaw, an understanding of transducer position and location on the scan surface is imperative to position the flaw on the inside surface. To accurately measure transducer location in azimuth around the nozzle circumference, to measure the transducer radial position with respect to nozzle center and to measure the transducer skew with respect to nozzle position are time consuming and difficult. It is also difficult and time consuming to lay out a nozzle for examination because of its geometry and the multiple search units that are used for specific radial areas around the nozzle. Each search unit is used in a specific radial position and the technician needs to be able to quickly identify the different areas.
Given that the nozzle, pressure vessel, and blend may be located in a radiation area, and that a significant portion of time spent in that environment is necessarily allocated to performance of the testing regime, there is a need for a device capable of accurately and quickly measuring the location and position (i.e. the coordinates and skew) of a flaw indication marking or tag, such as a traced outline, on the exterior of the nozzle, pressure vessel, and blend. Such a device should be capable of quickly measuring both the radial position and the angular position of a flaw indication (i.e., traced outline) relative to the axis of the nozzle, and the skew of the transducer probe which provided the flaw indication (traced outline).